Optical semiconductor elements such as LEDs (Light Emitting Diode) have been used as various indicators and light sources such as displays on the streets, automobile lamps and residential lightings. In fact, products employing optical semiconductor elements are being developed rapidly in various fields for the purpose of reducing carbon dioxide and saving energies.
An encapsulation material for encapsulating various optical semiconductor elements such as LEDs has to have a transparency, a moisture resistance, a heat resistance and a light resistance. Thus, as such encapsulation material, often used is a heat-curable epoxy resin (Patent document 1) employing both an epoxy resin such as a bisphenol A-type epoxy resin and/or an alicyclic epoxy resin; and an acid anhydride-based curing agent.
However, as a result of using a multifunctional epoxy resin and/or an alicyclic epoxy resin by simply melting the same to improve a heat resistance and a light resistance, a strength thereof as an encapsulation resin will decrease easily such that resin crack will occur easily as well if using such a kind of epoxy resin composition to encapsulate an optical semiconductor element (Patent documents 2 and 3).
Further, a lamp for use in an automobile or the like may be manufactured as follows. That is, an epoxy resin is used to encapsulate an optical semiconductor element on a lead frame in a lenticular manner, followed by proceeding to secondary molding where injection molding is performed on such lead frame with the aid of a thermoplastic resin. In such case, since the thermoplastic resin can only be molded when heated to 300° C. and under a high pressure, a low glass-transition temperature of the epoxy resin may cause the epoxy resin to reach a rubber state early such that the epoxy resin that has been poured into the lenticular section may be either washed away or separated from the lead frame.